Liquid membranes combine extraction and stripping, which are normally carried out in two separate steps in conventional processes such as solvent extractions, into one step. A one-step liquid membrane process provides the maximum driving force for the separation of a targeted species, leading to the best clean-up and recovery of the species (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992).
There are two types of liquid membranes: (1) supported liquid membranes (SLMs) and (2) emulsion liquid membranes (ELMs). In SLMs, the liquid membrane phase is the organic liquid imbedded in pores of a microporous support, e.g., microporous polypropylene hollow fibers (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992). When the organic liquid contacts the microporous support, it readily wets the pores of the support, and the SLM is formed.
For the extraction of a target species from a feed solution, the organic-based SLM is placed between two aqueous solutions--the feed solution and the strip solution--where the SLM acts as a semi-permeable membrane for the transport of the target species from the feed solution to the strip solution. The organic liquid in the SLM is immiscible in the aqueous feed and strip streams and contains an extractant, a diluent which is generally an inert organic solvent, and sometimes a modifier.
The use of SLMs to remove metals from aqueous feed solutions has been long pursued in the scientific and industrial community. The removal of metals, including cobalt, copper, nickel, zinc, cadmium, and gallium, from aqueous solutions has been studied (R. S. Juang and J. D. Jiang, "Rate-controlling Mechanism of Cobalt Transport through Supported Liquid Membranes Containing Di(2-ethylhexyl) Phosphoric Acid," Sep. Sci. Technol., 29, 223-237 (1994); T. Saito, "Selective Transport of Copper(I, II), Cadmium(II), and Zinc(II) Ions through a Supported Liquid Membrane Containing Bathocuproine, Neocuproine, or Bathophenanthroline," Sep. Sci. Technol., 29, 1335-1346 (1994); M. Teramoto, N. Ohnishi, and H. Matsuyama, "Effect of Recycling of Feed Solution on the Efficiency of Supported Liquid Membrane Module," Sep. Sci. Technol., 29, 1749-1755 (1994); F. F. Zha, A. G. Fane, and C. J. D. Fell, "Liquid Membrane Processes for Gallium Recovery from Alkaline Solutions," Ind. Eng. Chem. Res., 34, 1799-1809 (1995); S. B. Kunungo and R. Mohapatra, "Coupled Transport of Zn(II) through a Supported Liquid Membrane Containing bis(2,4,4-Trimethylpentyl) Phosphinic Acid in Kerosene. II Experimental Evaluation of Model Equations for Rate Process under Different Limiting Conditions," J. Membrane Sci., 105, 227-235 (1995)).
The extraction of rare earth metals, including europium, lanthanum, and neodymium, with SLMs has been investigated (C. Nakayama, S. Uemiya, and T. Kojima, "Separation of Rare Earth Metals Using a Supported Liquid Membrane with DTPA," J. Alloys Compounds, 225, 288-290 (1995); R. S. Juang and S. H. Lee, "Analysis of the Transport Rates of Europium(III) across an Organophosphinic Acid Supported Liquid Membrane," J. Membrane Sci., 110, 13-23 (1996)).
Recently, the removal of metals, including copper, zinc, cadmium, and palladium, with SLMs has been described (N. Aouad, G. Miquel-Mercier, E. Bienvenue, E. Tronel-Peyroz, G. Jerninet, J. Juillard, and P. Seta, "Lasalocid (X537A) as a Selective Carrier for Cd(II) in Supported Liquid Membranes," J. Membrane Sci., 139, 167-174 (1998); J. A. Daoud, S. A. El-Reefy, and H. F. Aly, "Permeation of Cd(II) Ions through a Supported Liquid Membrane Containing Cyanex-302 in Kerosene," Sep. Sci. Technol., 33, 537-549 (1998); J. Vander Linden and R. F. De Ketelaere, "Selective Recuperation of Copper by Supported Liquid Membrane (SLM) Extraction," J. Membrane Sci., 139, 125-135 (1998); M. E. Campderros, A. Acosta, and J. Marchese, "Selective Separation of Copper with LIX 864 in a Hollow Fiber Module," Talanta, 47, 19-24 (1998); M. Rovira and A. M. Sastre, "Modelling of Mass Transfer in Facilitated Supported Liquid-Membrane Transport of Palladium(II) Using Di-(2-ethylhexyl) Thiophosphoric Acid," J. Membrane Sci., 149, 241-250 (1998); J. C. Lee, J. Jeong, J. T. Park, I. J. Youn, and H. S. Chung, "Selective and Simultaneous Extractions of Zn and Cu Ions by Hollow Fiber SLM Modules Containing HEH(EHP) and LIX84," Sep. Sci. Technol., 34, 1689-1701 (1999); F. Valenzuela, C. Basualto, C. Tapia, and J. Sapag, "Application of Hollow-Fiber Supported Liquid Membranes Technique to the Selective Recovery of a Low Content of Copper from a Chilean Mine Water," J. Membrane Sci., 155, 163-168 (1999); M. Oleinikova, C. Gonzalez, M. Valiente, and M. Munoz, "Selective Transport of Zinc through Activated Composite Membranes Containing Di(2-ethylhexyl) Dithiophosphoric Acid as a Carrier," Polyhedron, 18, 3353-3359 (1999)).
The extraction of rare earth metals, including europium, lanthanum, neodymium, praseodymium, and gadolinium, with SLMs has been reported recently (M. R. Yaftian, M. Burgard, C. B. Dieleman and D. Matt, "Rare-earth Metal-ion Separation Using a Supported Liquid Membrane Mediated by a Narrow Rim Phosphorylated Calix[4]arene," J. Membrane Sci., 144, 57-64 (1998)).
One disadvantage of SLMs is their instability due mainly to loss of the membrane liquid (organic solvent, extractant, and/or modifier) into the aqueous phases on each side of the membrane (A. J. B. Kemperman, D. Bargeman, Th. Van Den Boomgaard, H. Strathmann, "Stability of Supported Liquid Membranes: State of the Art," Sep. Sci. Technol., 31, 2733 (1996); T. M. Dreher and G. W Stevens, "Instability Mechanisms of Supported Liquid Membranes," Sep. Sci. Technol., 33, 835-853 (1998); J. F. Dozol, J. Casas, and A. Sastre, "Stability of Flat Sheet Supported Liquid Membranes in the Transport of Radionuclides from Reprocessing Concentrate Solutions," J. Membrane Sci., 82, 237-246 (1993)). The prior art has attempted to solve this problem through the combined use of SLM with a module containing two sets of hollow fibers, i.e., the hollow-fiber contained liquid membrane (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992). In this configuration, with two sets of microporous hollow-fiber membranes, one set of membranes carries the aqueous feed solution, and the other carries the aqueous strip solution. The organic phase is contained between the two sets of hollow fibers by maintaining the aqueous phases at a higher pressure than the organic phase. The use of the hollow-fiber contained liquid membrane increases membrane stability because the liquid membrane may be continuously replenished. However, this configuration is not advantageous because it requires mixing two sets of fibers to achieve a low contained liquid membrane thickness.
In ELMs, an emulsion acts as a liquid membrane for the separation of the target species from a feed solution. An ELM is created by forming a stable emulsion, such as a water-in-oil emulsion, between two immiscible phases, followed by dispersion of the emulsion into a third, continuous phase by agitation for extraction. The membrane phase is the oil phase that separates the encapsulated, internal aqueous droplets in the emulsion from the external, continuous phase (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992). The species-extracting agent is contained in the membrane phase, and the stripping agent is contained in the internal aqueous droplets. Emulsions formed from these two phases are generally stabilized by use of a surfactant. The external, continuous phase is the feed solution containing the target species. The target species is extracted from the aqueous feed solution into the membrane phase and then stripped into the aqueous droplets in the emulsion. The target species can then be recovered from the internal, aqueous phase by breaking the emulsion, typically via electrostatic coalescence, followed by electroplating or precipitation.
The use of ELMs to remove metals from aqueous feed solutions has also been long pursued in the scientific and industrial community. ELMs for the removal of metals, including cobalt, copper, zinc, nickel, mercury, lead, cadmium, silver, europium, lanthanum, and neodymium, have been described in detail (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992). The removal of cobalt, copper, and nickel from aqueous solutions by ELMs has also been investigated (J. Strzelbicki and W. Charewicz, "The Liquid Surfactant Membrane Separation of Copper, Cobalt and Nickel from Multicomponent Aqueous Solutions," Hydrometallurgy, 5, 243-254 (1980)). The extraction of lanthanoids, including europium, lanthanum, neodymium, and gadolinium, with ELMs has been studied (M. Teramoto, T. Sakuramoto, T. Koyama, H. Matsuyama, and Y. Miyake, "Extraction of Lanthanoids by Liquid Surfactant Membranes," Sep. Sci. Technol., 21, 229-250 (1986); C. J. Lee, S. S. Wang, and S. G. Wang, "Extraction of Trivalent Europium via Emulsion Liquid Membrane Containing PC-88A as Mobile Carrier, Ind. Eng. Chem. Res., 33, 1556-1564 (1994); S. A. El-Reefy, M. R. El-Sourougy, E. A. El-Sherif, and H. F. Aly, "Europium Permeation and Separation from Americium Using Liquid Emulsion Membrane," Anal. Sci., 11, 329-331 (1995)).
Recently, the removal of metals including cobalt, nickel, cadmium, mercury, and lead with ELMs has been reported (M. Samar, D. Pareau, G. Durand, and A. Chesne, "Purification of Waste Waters Containing Heavy Metals by Surfactant Liquid Membrane Extraction," in Hydrometall. '94. Pap. Int. Symp., Chapman & Hall, London, UK, 1994, pp. 635-654; B. Raghuraman, N. Tirmizi, and J. M. Wiencek, "Emulsion Liquid Membranes for Wastewater Treatment. Equilibrium Models for Some Typical Metal-Extractant Systems," Environ. Sci. Technol., 28, 1090-1098 (1994); M.T.A. Reis and J. M. R. Carvalho, "Recovery of Heavy Metals by a Combination of Two Processes: Cementation and Liquid Membrane Permeation," Minerals Eng., 7, 1301-1311 (1994); T. Kakkoi, M. Goto, K. Sugimoto, K. Ohto, and F. Nakashio, "Separation of Cobalt and Nickel with Phenylphosphonic Acid Mono-4-tert-octylphenyl Ester by Liquid Surfactant Membranes," Sep. Sci. Technol., 30, 637-657 (1995); R. S. Juang and J. D. Jiang, "Recovery of Nickel from a Simulated Electroplating Rinse Solution by Solvent Extraction and Liquid Surfactant Membrane," J. Membrane Sci., 100, 163-170 (1995); B. J. Raghuraman, N. P. Tirmizi, B. S. Kim, and J. M. Wiencek, "Emulsion Liquid Membranes for Wastewater Treatment: Equilibrium Models for Lead- and Cadmium-di-2-ethylhexyl Phosphoric Acid Systems," Environ. Sci. Technol., 29, 979-984 (1995); E. Amanatidou, M. N. Stefanut, and A. Grozav, "Method of Cobalt Ion Concentration from Dilute Aqueous Solutions," Sep. Sci. Technol., 31, 655-664 (1996); Q. Li, Q. Liu, and X. Wei, "Separation Study of Mercury through an Emulsion Liquid Membrane," Talanta, 43, 1837-1842 (1997); H. Kasaini, F. Nakashio, and M. Goto, "Application of Emulsion Liquid Membranes to Recover Cobalt Ions from a Dual-component Sulphate Solution Containing Nickel Ions," J. Membrane Sci., 146, 159-168 (1998); Q. M. Li, Q. Liu, Q. F. Zhang, X. J. Wei, and J. Z. Guo, "Separation Study of Cadmium through an Emulsion Liquid Membrane Using Triisooctylamine as Mobile Carrier," Talanta, 46, 927-932 (1998); S. Y. B. Hu and J. M. Wiencek, "Emulsion-Liquid-Membrane Extraction of Copper Using a Hollow-Fiber Contactor," AIChE J., 570-581 (1998)).
One disadvantage of ELMs is that the emulsion swells upon prolonged contact with the feed stream. This swelling causes a reduction in the stripping reagent concentration in the aqueous droplets which reduces stripping efficiency. It also results in dilution of the target species that has been concentrated in the aqueous droplets, resulting in lower separation efficiency of the membrane. The swelling further results in a reduction in membrane stability by making the membrane thinner. Finally, swelling of the emulsion increases the viscosity of the spent emulsion, making it more difficult to demulsify. A second disadvantage of ELMs is membrane rupture, resulting in leakage of the contents of the aqueous droplets into the feed stream and a concomitant reduction of separation efficiency. Raghuraman and Wiencek (B. Raghuraman and J. Wiencek, "Extraction with Emulsion Liquid Membranes in a Hollow-Fiber Contactor," AIChE J., 39, 1885-1889 (1993)) have described the use of microporous hollow-fiber contactors as an alternative contacting method to direct dispersion of ELMs to minimize the membrane swelling and leakage. The hollow-fiber contactors minimize mebrane swelling and leakage because they do not have the high shear rates typically encountered with the agitators used in the direct dispersion. Additional disadvantages of ELMs include the necessary process steps for making and breaking the emulsion.
Thus, there is a need in the art for an extraction process which maximizes the stability of the SLM membrane, resulting in efficient removal and recovery of metals from the aqueous feed solutions.